In recent years, as the seawater desalination market has been expanding due to global water shortage, seawater desalination plants are being constructed. As a technology for seawater desalination, there is known a method for producing fresh water by removing salt in seawater with a reverse osmosis membrane (RO membrane). A filtration apparatus using an RO membrane performs removal of suspended matters as a pretreatment.
In order to remove suspended matters, in general, a flocculant is continuously injected into the seawater to flocculate the suspended matters. As the flocculant, iron salt is used. This metal reacts with an alkaline component in the water to generate metal hydroxide.
The metal hydroxide acts as a binder, and collision and contact of suspended matters in the seawater cause conglomeration, generating flocs. An injection amount of the flocculant is increased and decreased in accordance with an amount of suspended matters in the seawater. For example, when iron salt is used as the flocculant, the iron salt is injected so as to be 0.5 to 10 ppm as iron in the seawater.
Other methods for separating suspended matters include filter filtration, centrifugation, and filtration using a solid filter material. A method using a solid filter material is advantageous in that it is inexpensive as compared with filter filtration or centrifugation, and easy to maintain. For the solid filter material, those sized to have a diameter of 300 to 2500 μm are typically used. When suspended matters to be removed are small, the flocculant is added to water to be treated to form flocs thereby to increase the size of an object to be removed, and then the filtration is performed. Here again, the flocculant is continuously injected to the water to be treated.
Continuous injection of the flocculant causes growth of the flocs, which makes it easier to capture the flocs with a downstream filter. However, the filter itself must be washed regularly to discharge flocs that have been deposited inside, to outside of the system. The flocs deposited in the filter are discharged from inside of the filter by backwashing.
As a method using a solid filter material, there is also known a method that utilizes a biofilm to separate suspended matters with a filter filled with a solid filter material without using a flocculant, as disclosed in NPL 1.